<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
  <style>
    #box{
      width: 300px;
      border: 1px solid red;
      margin: 0 auto;
    }
    h2{
      background: green;
      margin: 0;
    }
    form{
      background-color: aqua;
    }
    input{
      border: 2px solid black;
    }
  </style>
</head>
<body>
<div id="box">
  <h2>会员登录</h2>
  <form action="#">
    <div>
      <span>用户名：</span>
      <input type="text"></input>
    </div>
    <div>
      <span>密码：</span>
      <input type="text"></input>
    </div>
    <div>
      <span>邮箱：</span>
      <input type="text"></input>
    </div>
  </form>
</div>
</body>
</html>